1. do you read somewhere, that the image doesn't fit the frame? or is there a visible white space?when i uncheck the 'proportional' box, my images fill the frames always (but there aren't many use cases for unproportionally stretched images in my work…)

2. afaik word never produced cmyk output. (at least not back in the days when i used it…) … and a .pdf/x-3 veryfier shouldn't report an error for a mix of rgb and cmyk color spaces, as this is perfectly ok with the .pdf/x-3 specs.

if your printshop requires .pdf/x-3 files with all elements converted to cmyk, you should export as .pdf/x-1a!